Radioactivity well logging for deter-mining the presence of hydrogen



March 15, 1966 A, s. M KAY ETAL 3,240,937

RADIOACTIVITY WELL LOGGING FOR DETERMINING THE PRESENCE OF HYDROGENOriginal Filed June 15, 1959 2 Sheets-Sheet 1 Tlal. 45' /9 20 March 15,1966 o Y ETAL 3,240,937

RADIOACTIVITY WELL LOGGING FOR DETERMINING THE PRESENCE OF HYDROGENOriginal Filed June 15, 1959 2 Sheets-Sheet z United States Patent3,240,937 RADIOACTIVITY WELL LOGGING FOR DETER- MINING THE PRESENCE OFHYDROGEN Alexander S. McKay, Hopewell Junction, N.Y., and Hugh E. Hall,Jr., Houston, Tex., assignors to Texaco Inc., New York, N.Y., acorporation of Delaware Continuation of application Ser. No. 820,240,June 15, 1959. This application June 14, 1963, Ser. No. 289,163 29Claims. (Cl. 250-71.5)

The present invention relates generally to the determination of thenature of earth formations; and, more particularly, it is concerned withthe analysis of earth formations along the traverse of a bore holethrough irradiation of the formations with neutrons in order to producecertain observable effects that are detected as an indication of thenature of the formations.

This application is a continuation of application Serial No. 820,240,filed June 15, 1959, now abandoned.

The invention is especially directed toward improvements inneutron-gamma ray well logging for quantitatively determining in situthe presence of hydrogen in earth formations traversed by a bore hole.Accordingly, it is a general object of the present invention to provideimprovements in radio-activity well logging wherein a source of neutronsis employed to bombard earth formations in order to cause the emissionof gamma rays which are detected and whose intensity is measured as anindication of the hydrogen content of the earth formations along thewell bore.

It is well known to analyze earth formations in situ along the traverseof a bore hole through the use of various radioactivity analysistechniques. It is possible to determine the presence of porous zonesalong the path of the bore hole through the use of such techniques whichmay indicate the presence or absence of hydrogen in the pores of theformation, which hydrogen may be present in hydrocarbon material aseither oil or gas or as a constituent of water.

In accordance with the so-called neutron-gamma ray logs, a source ofneutrons is passed through the bore hole in order to irradiate the earthformations along the traverse of the bore hole. The neutrons from thesource are slowed down in the formation and bore hole fluid, primarilydue to the effect of hydrogen, and after being slowed to the thermalrange the neutrons are captured by material of the formation with theresultant emission of gamma rays. These neutron-gamma rays which aredetected and their intensity, i.e., rate-of-occurence is measured as ameasure of the hydrogen content of the formations.

When the thermalized neutrons are captured by hydrogen, gamma rayshaving a characteristic energy of 2.2 million electron volts are emittedby the hydrogen responsible for the capture. When only hydrogen ispresent in the pores of the region under investigation, the intensity,i.e., rate of occurrence of the detected gamma radiation provides a goodquantitative indication of the hydrogen content of the formation.However, it has been determined that other materials which may also bepresent in the formation can have an adverse effect on the neutron-gammaray well log and which may render the log unreliable as a hydrogenmeasurement. Of particular significance is the presence of chlorinewhich has a relatively high capture cross-section for thermal neutrons,as compared with that of hydrogen. In particular, chlorine has a neutroncapture cross-section of about 32 Barns, whereas hydrogen has a neutroncapture crosssection of about .33 Barns. Thus, chlorine is approximately100 times more effective in capturing thermal neutrons than hydrogen.When a thermal neutron is captured by chlorine, rather than hydroigen,about 3.1 gam Ina rays (on the average) are emitted per capture ascompared with one gamma ray per capture by a hydrogen atom. In additionto the foregoing, many of the gamma rays emitted by chlorine are ofhigher energy range, from about 4-8 mev., than the characteristic 2.2mev. gamma rays of capture emitted by hydrogen. In view of the foregoingthe presence of even a small amount of chlorine will ordinarily increasethe intensity of the gamma radiation detected by the neurton-gamma rayinstrument, thus giving a false indication of hydrogen content in theresultant log.

Accordingly, it is a more specific objective of the pressent inventionto overcome the adverse effect of the presence of chlorine onneutron-gamma ray well logs for determining the hydrogen content ofearth formations along the traverse of the well bore.

Briefly stated, one aspect of the present invention involves an improvedmethod of neutron-gamma ray well logging wherein the adverse effect ofchlorine. upon the detected gamma ray signal is neutralized bysubjecting the gamma ray detector to radiation, the intensity of whichvaries inversely with the intensity of the gamma radiation due to thepressure of chlorine, whereby the effect of chlorine is effectivelyneutralized.

A preferred aspect of the present invention involves the provision of animproved well logging instrument adapted to be passed through a borehole and which comprises a source of neutrons for irradiating theformations traversed by the bore hole and a gamma ray detector fordetecting radiation indicative of the presence of hydrogen in theformation, in order to provide a signal display of the intensity of thedetected radiation in correlation with the position of the instrument inthe bore hole, which improvement involves the provision in the vicinityof the detector of a predetermined quantity of neutron absorbingmaterial characterized by the emission of radiation to which the saiddetector is sensitive. Preferably, the invention involves the provisionof a gamma ray detector surrounded by a predetermined quantity ofcadmium.

Advantageously, the position of the logging instrument is stabilized inthe bore hole in order to avoid variations in neutron-gamma ray signalwhich might otherwise result. A fixed position centralized ordecentralized, is important because, if the tool is not positioned,there is no way of knowing whether the changes in the response are dueto hydrogen content changes of the formation or due to variations in theposition.

For additional objects and advantages and for a better understanding ofthe invention, attention is now directed to the following descriptionand accompanying drawings. The features of the invention which arebelieved to be novel are particularly pointed out in the appendedclaims.

In the drawings:

FIG. 1 is a schematic representation showing a vertical elevationthrough a portion of a bore hole having a well logging instrumentsuspended therein and which is constructed in accordance with theprinciples of the invention; and,

FIG. 2 is a schematic representation similar to FIG. 1 and showinganother form of logging instrument embodying principles of theinvention.

Referring now to FIG. 1 of the drawings, there is shown a bore hole 10transversing a plurality of earth formations 11, 12, 113 and 14 andcontaining a flui d 15 which may comprise the usual drilling fluid,water or crude oil, for example. 10 as by means of a cable 16, there isshown a well logging instrument 17 constructed in accordance withprinciples of the invention. The cable 16 may include an outerconductive sheath together with one or more addi- Suspended within thebore hole' 3 tional inner conductors (not individually shown) in orderto afford means for transmitting electrical signals between theinstrument 17 and electrical apparatus at the surface of the earth.

The surface equipment provides a means for receiving signals transmittedfrom the logging instrument 16 and amplifying and otherwise translatingthe received signals as necessary for recording purposes. The surfaceapparatus includes an amplifier 18 having its input side connected tothe cable 16 and having its output side connected to a pulse heightanalysis or discriminator 19. The output path from the discriminator 19leads to a display device in the the form of a recorder 20. It is to beunderstood that the recorder 20 includes rate meter circuitry asnecessary for providing a record of the intensity, i.e.,rate-of-occurrence of detected radiation. It is also to be understoodthat although the discriminator 19 is shown as part of the surfaceequipment, it may preferably be included as part of the equipmentcontained within the logging instrument 17.

In order to correlate the position of the logging instrument 17 in thebore hole during a well log, there is provided at the surface ameasuring apparatus 21, represented diagrammatically as a wheel havingits perimeter in contact with the cable 16 for sensing movement of thecable 16 in and out of the bore hole 10. The measuring apparatus 21 maybe any known device of this type suitable for determining the positionof the logging instrument 17 in the bore hole 10 and advantageously maybe of a type which provides an electrical output signal which may betransmitted as by means of the conductive circuit 22 to the recorder forcorrelating the recorded logging signal with the position of the logginginstrument 17 in the bore hole 10 throughout a well log.

Logging instrument 17 comprises an elongated outer shell-like housing orcasing 23 formed conventionally of steel in accordance with knowntechniques to withstand the pressures and temperatures commonlyencountered in the Well logging art. Advantageously, the housing shouldbe of such character as to withstand the conditions that may be found ina bore hole upwards of 10,000 to 20,000 feet in depth. The casing 23contains a neutron source 24 for bombarding the earth formations alongthe bore hole, together with appropriate radiation detection equipmentfor detecting gamma rays induced in the formation as the result ofirradiation by the source 24. Appropriate electrical circuitry is alsoenclosed within the casing 23 for amplifying and otherwise handling theoutput signals from the radiation detection equipment for transmissionover the cable 16 to the surface equipment. In particular, the neutronsource 24 is shown positioned within the housing 23 near the lower endthereof and having positioned immediately above it, a gamma ray shield25 formed of lead or tungsten, for example, to prevent gamma radiationwhich may also be emitted by the source from passing to the detectionequipment. Also within the housing 23 above the shield 25 and spaced apredetermined distance from the source 24, the-re is positioned aradition detection unit 26 which, together with its associatedcircuitry, is adapted and arranged to provide an output signal that isproportional to the intensity of detected gamma radiation resulting fromneutrons emitted by the source. The detection unit 26 is constructed inaccordance with principles of the inven tion so that it provides anoutput signal which is significantly proportional to the hydrogencontent of the earth formation bombarded by neutrons from the source.

The detector unit 26 comprises a scintillation detector including agamma ray sensitive luminophor 27, advantageously in the form of asodium iodide crystal, together with a photomultiplier tube 28, shownmounted adjacent the luminophor 27 for detecting the output pulses fromthe luminophor and providing an electrical signal proportional thereto.The photomultiplier tube 28 is, in turn, shown adjacent a preamplifier29, identified schematically and, in turn, is shown electricallyconnected to additional electrical equipment identified as an amplifier30 for transmission of output signals derived from the photomultipliertube 28 to the surface equipment by means of the cable 16. It is to beunderstood that the photomultiplier tube is energized by means of a highvoltage source (not shown) which may comprise batteries positioned inthe logging instrument 17, or, more conventionally, may involve the useof power supply equipment (not shown) including a transformer andrectifier in the logging instrument for deriving appropriate high directcurrent operating potential from alternating power transmitted from thesurface to the logging instrument in the bore hole. It is to be furtherunderstood that the amplifier 30 may actually include any additionalcircuitry required for handling the transmission of the signalinformation to the surface in accordance with the principles well knownin the art. For example, the radiation detector signals may betransmitted to the surface as amplitude modulated signals or may betransmitted in the form of frequency modulated signals in accordancewith well known techniques. As is well known to those skilled in theart, filter means may also be provided for separating electrical powertransmitted from the surface to the equipment in the logging instrumentfrom the electrical signals which are transmitted from the logginginstrument to the surface equipment.

The luminophor 27 is shown mounted within an aluminum container 31 whichserves to protect it from moisture and physical damage. The upper end ofthe container 31 facing the photomultiplier tube 28 is shown as open,with the luminophor 27 in direct contact with the sensitive face of thephotomultiplier tube 28. However, it is to be understood that thecontainer 31 may be sealed with a transparent cover of glass or plasticin a manner well known in the art. Surrounding the container 31 there isprovided a thin layer 32 of a neutron-absorbing material which emits aplurality of neutron-capture gamma radiations which as hereinafterdiscussed functions in combination with other elements of the inventionto render the neutron-gamma ray signal derived by the gamma ray detectorprimarily sensitive to hydrogen and relatively insensitive to the effectof chlorine.

In order to stabilize and protect the scintillation logging equipmentagainst the effect of high bore hole temperatures, and variationsthereof, the luminophor and photomultiplier tube, as well as thepreamplifier are all shown mounted within an insulated chamberpreferably in the form of Dewar flask comprising an outer wall 33separated from an inner wall 34 by an evacuated space. The insulatedchamber is provided with an appropriate removable insulating plug 35 ofcork or other insulating material. Within the Dewar flask advantageouslyaffixed to the inside of the insulated cover plug 35, there is provideda coolant chamber 36 having thermally conductive walls, as of thinaluminum, and containing a quantity of ice 37. The coolant chamberincluding the ice 37 affords means for maintaining the scintillationdetection equipment in a stable, relatively low temperature environmentby virtue of the temperature stability afforded as the ice undergoes achange of state from the solid to the liquid phase during the loggingoperation. It is to be understood that other techniques for stabilizingthe temperature of the instrument may be employed, for example, as shownand described in US Letters Patent No. 2,824,233, granted February 18,1958 to Gerhard Herzog.

Means are provided to stabilize the position of the logging instrumentthroughout a logging run, comprising a decentralizing bow spring 40,having its upper and lower ends 41 and 42 mounted to the logginginstrument 17 in such manner that the spring is free to flex. as theinstument is moved past irregularities in the side of the bore hole.conventionally, this may be achieved by mounting the upper and lowerends of the bow spring 40 in slidably engageable relationship to theinstrument 17, as by means of elongated slots (not shown) in the bowspring 40.

In order to insure that the hydrogen detector 26 is responsive to thehydrogen only and not the natural gamma radiation or scattered gammaradiation from the neutron source, the discriminator associatedtherewith should advantageously be biased to exclude these undesirablelower energy gamma rays. Preferably, the discriminator should be biasedso that the measured radiation signal is indicative of gamma rays havingan electron energy of about 1.8 million electron volts and above as willbe discussed later. This particular bias level, in combination with the2" X 4" sodium iodide crystal and a cadmium layer for the neutronabsorbing material 32 which surrounds the luminophor 27 at asource-to-detector spacing of 16 inches operates to provide a verysatisfactory logging signal which is primarily responsive to thehydrogen content of the formations, due to radiation resulting fromirradiation of the formation by neutrons from the source 24. By thusbiasing the detector to exclude lower energy gamma rays, most of thenatural gamma radiation present in the formation and any gamma radiationscattered from the source is likely to be eliminated from the detectedsignal, since the natural and scattered gamma radiations are both ofrelatively low energy level.

The neutralization of the adverse effect of chlorine upon a neutrongamma ray well log is accomplished in accordance with the presentinvention by subjecting the gamma ray detector to radiation which variesinversely with the effect upon the detector due to the presence ofchlorine. As discussed above, chlorine has a relatively high capturecross-section and emits a plurality of gamma rays in response to thecapture of each neutron. Thus, the counting rate of the gamma raydetector is increased due to the presence of chlorine. In accordancewith the present invention a gamma ray signal is developed whoseintensity decreases correspondingly due to the presence of chlorine.This may be accomplished by developing a gamma ray signal whoseintensity is proportional to the thermal neutron flux in the vicinity ofthe detector. This is due to the fact that chlorine absorbs or removesthermal neutrons from the environment of the detector due to itsrelatively high capture cross-section. Thus, the thermal neutron flux inthe vicinity of the detector is reduced due to the presence of chlorine.By introducing a neutron-absorbing material which emits a plurality ofneutron capture gamma radiations in the vicinity of the detector,thermal neutrons are absorbed in said material which result in theemission of gamma radiation which is detected by the gamma ray detector.Using a sodium iodide detector, the thickness of cadmium needs to be atleast enough to absorb most of the thermal neutrons and the balance isachieved by adjusting the discriminator bias. When the logginginstrument passes into a region containing chlorine from an identicalregion with the exception that no chlorine is present, the counting rateof the gamma ray detector tends to increase due to the increase of thenumber of capture gamma rays directly attributable to the chlorine.However, when this occurs, the number of thermal neutrons available forcapture by the neutron absorbing material around the detector is reducedthus tending to decrease the counting rate in the gamma ray detector. Bysuitable means, these two effects may be made to cancel one another, sothat when a logging tool using this system is passed from a formationcontaining chlorine to one not containing chlorine but have the sameporosity, formation matrix, and hydrogen content, the response remainsconstant.

In the apparatus shown in FIG. 1, the above-mentioned two effects may bemade equal and opposite either by adjusting the bias of thediscriminator, by adjustment of the amount of the neutron absorbingmaterial 32, or by a combination. For example, the neutron absorbingmaterial 32 may be a sheet of cadmium surrounding the crystal at such athickness, i.e., .025" that it essentially captures all of the thermalneutrons which dilfuse to the layer of cadmium. The rise in the capturegamma component may be balanced against the thermal neutron component bya discriminator setting of approximately 1.8 mev. for asource-to-detector spacing of 16". The discriminator bias will be in therange from 1 to 2.5 mev. depending upon the detector-to-source spacing,the dimension of the crystal, the case thickness, the case material, thediameter of the bore hole and the salinity of the fluid.

At the lower bias settings, there is more thermal neutron component thanis necessary, and at higher bias settings there is less than is needed.Thus, in accordance with one aspect of the invention the bias may beset, for example, at 1.0 mev. and the thickness of cadmium adjusted sothat the two effects cancel. If this were done, one would wish to placean additional neutron absorbing material between the crystal and thecadmium, such as boron or lithium, sufliciently thick to absorb thethermal neutrons transmitted through the cadmium. This would bedesirable since neither boron nor lithium emit neutron capture gammarays above 1.0 mev. and the crystal would not become activated. Thisaspect is illustrated in FIG. 1a which shows a portion of the apparatusof FIG. 1, as modified by the addition of a layer of neutron capturingmaterial 38, such as boron, provided between the neutron interactionmaterial 32 and the luminophor 27.

While cadmium is preferred as the material for capturing the thermalneutrons in the vicinity of the detector in order to neutralize theeifect of chlorine upon the neutron gamma ray log, it is to beunderstood that other materials may be employed rather than cadmium andthat other materials may be employed together with cadmium. In thepresent case, where the cadmium is employed in a logging instrumenthaving a steel instrument casing or housing, the iron of the logginginstrument coperates with the cadmium. Iron has a much lower capturecross-section for neutrons than cadmium, however, there is a relativelylarge quantity of iron present in the vicinity of the detector, hence,its effect is similar to that of the cadmium. Iron has a thermal neutroncapture cross-section of 2.43 Barns compared with a capturecross-section of about 3500 Barns for cadmium. Iron emits neutroncapture gamma rays ranging up to 9.3 mev., whereas cadmium emits gammasup to 9.05 mev. upon capture of thermal neutrons. Gadolinium may also beemployed in carrying out the invention. Gadolinium has a capturecross-section of 36,300 Barns and emits neutron capture gammas up to7.78 mev. As mentioned above, chlorine, the effect of which is to bebalanced out of the resultant detected gamma ray signal, has a capturecross-section of about 32 Barns and produces neutron capture gamma raysup to about 8.56 mev.

Whether cadmium or other material having similar characteristics or acombination of such materials is employed, such as the combination ofcadmium with the steel logging casing, it is important that the sumtotal of such material used having a predetermined net effect whichresults in the development of gamma ray signal in response to thethermal neutron population in the immediate vicinity of the detectorwhich just cancels out the capture gamma eifect due to the presence ofchlorine in the bore hole and formation in the vicinity of the detector.

In a preferred embodiment employing a neutron source comprising 200 mc.of Ra:Be, a sodium iodide crystal radiation detector of 4" length and 2"diameter biased at 1.8 mev., and having its near side spaced from theneutron source a distance of 16", it has been found that with a steellogging casing having a thickness of a cadmium sleeve greater than milsin thickness provides a very satisfactory cancellation of the adverseeffects of chlorine on the neutron-gamma ray log in formations havingaverage ranges of porosity and salinity which are 1030% porosity andsalinities from zero to saturation.

Referring now to FIG. 2, there is shown another embodiment of theinvention wherein the elements corresponding to those of the apparatusshown in FIG. 1 are identified with reference numerals having a value of100 plus the value of the numeral applied to the corresponding elementof FIG. 1. Thus, for example, the bore hole 10 and formations 11-14 ofFIG. 1 correspond to the bore hole 110 and formation 111-114,respectively, of FIG. 2. Whereas the apparatus of FIG. 1 discloses agamma ray detector of the scintillation type, the detector in apparatusof FIG. 2, involves the use of a high efiiciency electrical pulseproducing detector of the type disclosed in U.S. Patent No. 2,397,071 inthe name of D. G. C. Hare. This type detector may, for convenience, bereferred to as a high efficiency multiple plate cathode type Geigercounter and enables the constructure of a very reliable neutron gammaray well logging instrument embodying principles of the invention andwhich affords certain advantages as compared with apparatus involving ascintillation type detector.

The detector 150 shown in FIG. 2 is shown surrounded by a layer ofneutron absorbing material 132, discussed in detail below, and which is,in turn, surrounded by a lead shield 151 for excluding low energyscattered gamma radiation from the neutron source if a source emittingsubstantial gamma radiation is used. It has been found that this shieldshould have a gamma ray stopping power of A1" thick lead or equivalentfor an Ra:Be neutron source. The surface equipment shown in FIG. 2 doesnot include a discriminator such that shown in FIG. 1. By using the highefficiency pulse type detector, together with the disclosed leadshielding and an RazBe neutron source, the scattered gamma radiationsare eliminated without the need for the discriminator.

In the apparatus shown in FIG. 2, the detector is sensitive to gammaradiation having energies above a few kev. Therefore, in order to makethis log insensitive to the chlorine content in the volume surroundingthe detector, the capture gamma component of the response may bebalanced against the thermal neutron component only by varying theamount of neutron absorbing material 132 in the vicinity of thedetector. Various neutron aborbing materials 132 may be used aspreviously discussed. For example, a sheet of cadmium sufficiently thickto capture all of the thermal neutrons which get to the cadmium, placedaround a 2"x4 gamma radiation detector as previously described, overcompensates or, in other words, introduces a thermal neutron componentin the response which is too large. The amount of cadmium may be reducedslightly, i.e., by cutting holes in it, to afford the correct degree ofcompensation. To afford the correct degree of compensation,approximately of the thick cadmium sheet should be removed by cuttingsmall holes uniformly spaced over the detector. Alternatively one couldreduce the thickness of the cadmium sheet to approximately .0O4 toaccomplish the same purpose.

In the case of the apparatus shown in FIG. 2, it is considered moreadvantageous to use a neutron source that is relatively free of gammaray emission than in the case of the apparatus of FIG. 1. Accordingly,the neu tron source of the apparatus of FIG. 2, should in the preferredembodiment comprise a relatively gamma-free source of neutrons such asactinium 227 and beryllium or a comparable gamma-free source. In thecase of the apparatus of FIG. 1, a radium-beryllium, neutron source maybe employed together with the biasing discussed above, which iseffective to exclude the source-emitted gamma rays from the detectedgamma ray signal.

In accordance with a further aspect of the herein disclosed invention,it is contemplated that the neutron interaction material in the vicinityof the neutron-gamma ray detector may be placed around the outside ofthe casing of the logging instrument in the vicinity of the detector,rather than inside the casing as shown in the drawings. For example,this aspect may advantageously be carried out by plating the outside ofthe logging instrument in the vicinity of the neutron-gamma ray detectorwith a predetermined quantity of cadmium in order to provide foreffective neutralization of the effect of chlorine upon theneutron-gamma ray log, as discussed in detail hereinabove.

The subject matter of this application is closely related to certainsubject matter disclosed and claimed in applicants copendingapplications Serial Nos. 820,236, 820,237, now US. Patent No. 3,151,242,820,239, now US. Patent No. 3,147,378 and 820,241.

Obviously many modifications and variations of the invention, ashereinbefore set forth, may be made without departing from the spiritand scope thereof, and therefore, only such limitations should beimposed as are indicated in the appended claims.

We claim:

1. In the method of neutron-gamma ray well logging involving passing asource of neutrons through a bore hole to irradiate the formationstraversed by the bore hole and detecting neutron induced gamma raysemitted from the formation as a result of said irradiation and providinga signal display proportional to the intensity of the detected gammaradiation in correlation with the position of the detector in the borehole, the improvement wherein the effect of chlorine on the neutrongamma ray log is substantially neutralized by simultaneously detectingalong with said neutron-induced gamma rays a predetermined quantity of aradiation component whose intensity varies inversely with the variationsin the neutron-gamma ray signal as affected by chlorine.

2. In the method of neutron-gamma ray well logging involving passing asource of neutrons through a bore hole to irradiate the formationstraversed by the bore hole and detecting neutron-induced gamma raysresulting from said irradiation and providing a signal displayproportional to the intensity of the detected gamma radiation incorrelation with the postion of the detector in the bore hole as ameasure of a characteristic of the irradiated formations, theimprovement wherein the effect of chlorine on the neutron gamma ray logis substantially neutralized by simultaneously producing a predeterminedquantity of a radiation component whose intensity varies inversely withvariations in the neutron gamma ray signal as affected by chlorine anddetecting said component simultaneously with the neutron-induced gammarays.

3. In apparatus for conducting a neutron gamma ray well log of a borehole comprising a logging instrument adapted to be passed through a borehole and comprising a source of neutrons and a detector of gamma rayswhich may result from irradiation of an earth formation by neutrons fromthe source, the improvement comprising a predetermined quantity ofneutron aborbing material positioned in the vicinity of the detector andcharacterized by the emission of radiation to which the detector issensitive, said detector being substantially unshielded with respect tosaid radiation emitted from said neutron absorbing material such thatsaid detector is exposed to at least a major portion of said radiationemitted from said neutron aborbing material and travelling toward saiddetector, the quantity of said aborbing material being chosen inrelation to the intensity and spacing from said source such thatradiations will be emitted therefrom in response to interaction withneutrons having an intensity which substantially neutralizes the effectof chlorine on the neutron gamma ray log.

4. In apparatus for conducting a neutron gamma ray well log of a borehole comprising a logging instrument adapted to be passed through a borehole and comprising a steel housing containing a source of neutrons anda detector of gamma rays which may result from irradiation of an earthformation by neutrons from the source, the improvement comprising a thinsleeve surrounding at least a portion of the active volume of thedetector and formed of a predetermined quantity of neutron absorbingmaterial characterized by the emission of radiation to which thedetector is sensitive, said detector being substantially unshielded withrespect to said radiation emitted from said neutron absorbing materialsuch that said detector is exposed to at least a major portion of saidradiation emitted from said neutron absorbing material and travellingtoward said detector, the quantity of said absorbing material beingchosen in relation to the intensity and spacing from said source suchthat radiation will be emitted therefrom in response to interaction withneutrons having an intensity which substantially neutralizes the effectof chlorine on the neutron gamma ray log.

5. Apparatus as defined in claim 3 wherein the neutron absorbingmaterial is cadmium.

6. Apparatus as defined in claim 4 wherein the sleeve ofneutron-absorbing material is formed of cadmium.

7. In apparatus for conducting a neutron gamma ray well log of a borehole comprising a logging instrument adapted to be passed through a borehole and comprising a source of neutrons and a detector of gamma rayswhich may result from irradiation of an earth formation by neutrons fromthe source, the improvement comprising a scintillation type gamma raydetector having a predetermined quantity of neutron-absorbing materialpositioned in the vicinity of the detector and characterized by theemission of radiation to which the detector is sensitive, said detectorbeing substantially unshielded with respect to said radiation emittedfrom said neutron absorbing mate rial such that said detector is exposedto at least a major portion of said radiation emitted from said neutronabsorbing material and travelling toward said detector, the quantity ofsaid absorbing material being chosen in relation to the intensity andspacing from said source such that radiation will be emitted therefromin response to interaction with neutrons having an intensity whichsubstantially neutralizes the eifect of chlorine on the neutron gammaray log, said scintillation type detector having a pulse amplitudediscriminator associated therewith which is biased to exclude at leastthose lower energy gamma rays which may be scattered to the detectorfrom the source.

8. In apparatus for conducting a neutron gamma ray Well log of a borehole comprising a logging instrument adapted to be passed through a borehole and comprising a source of neutrons and a detector of gamma rayswhich may result from irradiation of an earth formation by neutrons fromthe source, the improvement comprising an electrical pulse producinggamma ray detector having operatively associated therewith apredetermined quantity of neutron-absorbing material characterized bythe emission of radiation to which the detector is sensitive, saiddetector being substantially unshielded with respect to said radiationemitted from said neutron absorbing material such that said detector isexposed to at least a major portion of said radiation emitted from saidneutron absorbing material and travelling toward said detector, thequantity of said absorbing material being chosen in relation to theintensity and spacing from said source such that radiation Will beemitted therefrom in response to interaction with neutrons having anintensity which substantially neutralizes the eiIect of chlorine on theneutron gamma ray log, said detector having a surrounding gamma rayshield to exclude lower energy gamma rays which may be scattered to thedetector from the source.

9. In apparatus for conducting a neutron gamma ray well log of a borehole to determine the hydrogen content of the formations along thetraverse of the bore hole comprising a logging instrument adapted to bepassed through a bore hole and comprising a source of neutrons and -adetector of gamma rays which may result from irradiation of an earthformation by neutrons from the source, the improvement comprising ascintillation type gamma ray detector having a predetermined quantity ofneutron-absorbing material positioned in the vicinity of the detectorand characterized by the emission of radiation to which the detector issensitive, said detector being substantially unshielded with respect tosaid radiation emitted from said neutron absorbing material such thatsaid detector is exposed to at least a major portion of said radiationemitted from said neutron absorbing material and travelling toward saiddetector, the quantity of said absorbing material being sufficientlylarge to capture essentially all thermal neutrons which diiIuse theretoas the result of said source, said material being characterized by theemission of radiation in response to interaction with neutrons having anintensity which tends to neutralize the effect of chlorine on theneutron gamma ray log, said scintillation type detector having a pulseamplitude discriminator associated therewith which is biased to excludethose lower energy gamma rays having an electron energy below apredetermined value such that the intensity of the radiation emitted bysaid substance substantially neutralizes the effect of chlorine on theneutron gamma ray hydrogen content log.

10. Apparatus according to claim 9 wherein said predetermined bias valueis in the range of 1.0 million electron volts to 2.5 million electronvolts.

11. In apparatus for conducting a neutron gamma ray well log of a borehole comprising a logging instrument adapted to be passed through a borehole for conducting a log of the hydrogen content of the formationstraversed by the bore hole and comprising a source of neutrons and adetector of gamma rays which may result from irradiation of an earthformation by neutrons from the source, the improvement comprising ascintillation type gamma ray detector including a luminophor having apredetermined quantity of a first type of neutron-absorbing materialpositioned in the vicinity thereof, said first type of material beingcharacterized by the emission in response to neutron absorption ofradiation to which the luminophor is sensitive, said detector beingsubstantially unshielded with respect to said radiation emitted fromsaid neutron absorbing material such that said detector is exposed to atleast a major portion of said radiation emitted from said neutronabsorbing material and travelling toward said detector, saidscintillation type detector having a pulse amplitude discriminatorassociated therewith which is biased to exclude gamma rays below apredetermined lower energy value of the order of one million electronvolts, the quantity of said first type of absorbing material :beingchosen in correlation with said bias value such that radiation reachingsaid luminophor therefrom in response to interaction with neutrons is ofan intensity which substantially neutralizes the effect of chlorine onthe neutron gamma ray log, and a layer of a second type of neutronabsorbing material substantially surrounding said luminophor, saidsecond type of material being sufiiciently thick to interceptsubstantially all thermal neutrons which may diffuse thereto as theresult of said source and which are not intercepted by said first typeof material, said second type of material being substantially permeableto gamma radiation and having a substantially zero emissioncharacteristic for neutrongamma rays having energies above said gammaray bias value.

12. Apparatus according to claim 11 wherein said first type of materialincludes at least one element from the group consisting of cadmium andgadolinium.

13. Apparatus according to claim 11 wherein said second type of materialis selected from the class of materials consisting of boron and lithium.

14. Apparatus according to claim 12 wherein said second type of materialis selected from the class of materials consisting of boron and lithium.

15. In the method of neutron-gamma ray well logging involving passing asource of neutrons through a bore hole to irradiate the formationstraversed by the bore hole and detecting neutron-induced gamma raysresulting from said irradiation and providing a signal displayproportional to the intensity of the detected gamma radiation incorrelation with the position of the detector in the bore hole as aquantitative indication of the hydrogen content of the irradiatedformations, the improvement wherein the effect of chlorine on theneutron-gamma ray log as a measure of the hydrogen content issubstantially neutralized in the signal display which comprises thesteps of intercepting a predetermined percentage of the thermal neutronsin the vicinity of the detector and producing a corresponding gamma rayflux which varies according to the intercepted thermal neutron flux anddetecting said corresponding gamma ray flux along with the neutrongammarays emitted from the formation as the result of said source.

16. The method of claim 15 which involves the use of a scintillationtype gamma ray detector having associated therewith a pulse amplitudediscriminator suitable to exclude gamma rays below a predetermned energylevel and which involves the additional step of adjusting the bias ofsaid discriminator until the effect of chlorine upon the neutron-inducedgamma ray hydrogen content signal is substantially neutralized by thegamma rays produced as a result of intercepting thermal neutrons in thevicinity of the detector.

17. The method of neutron-gamma ray well logging according to claim 1wherein the neutron gamma rays detected are those primarily indicativeof the presence of hydrogenous material whereby the signal displayproportional to the intensity of the detected gamma radiation providesan indication of the hydrogen content of the earth formations along thewell bore.

18. Apparatus as defined in claim 3 wherein the spacing between saidsource and said detector is chosen to provide a logging signal which isprimarily responsive to the hydrogen content of the formations logged.

19. Apparatus as defined in claim 18 wherein said spacing between thesource and detector is of the order of 16 inches.

20. In the method of radioactivity Well logging involving passing asource of neutrons through a bore hole to irradiate the formationstraversed by the bore hole and detecting first radiations occurring inthe bore hole as a result of said irradiation and providing a signaldisplay porportional to the intensity of said detected radiation incorrelation with the position of the detector in the bore hole as anindication of a characteristic of the formation, the improvement whereinthe effect of chlorine on the radiation log is substantially neutralizedby simultaneously detecting along with said first radiation apredetermined quantity of a second radiation whose intensity variesinversely with the variations in the intensity of said first radiationdue to the effect of the chlorine in the environment of said radiation.

21. The method of claim 20 wherein the intensity of said first radiationvaries as a function of both the hydrogen content and chlorine contentof its environment.

22. The method of claim 21 wherein the intensity of said secondradiation also varies as a function of the hydrogen content of theenvironment and wherein said variations in the intensity of said secondradiation due to hydrogen are inverse to said variations in theintensity of said second radiation due to the effect of chlorine.

23. The method of claim 22 wherein said first radiation comprisesessentially neutron-induced gamma radiation and said second radiationcomprises essentially neutron radiation.

24. In a system for conducting a radioactivity well log comprising alogging instrument adapted to be passed through a bore hole traversing aplurality of earth formations, said instrument including meanscomprising a source of neutrons for irradiating earth formations alongthe traverse of the bore hole, radiation detecting means quantitativelysensitive to the intensity of radiations occurring in the bore hole as aresult of the irradiation of the earth formations by neutrons from saidsource, said detecting means comprising means for selectively detectingthe combination of a first radiation component resulting from saidirradiation which varies in intensity as a function of said givencharacteristic and which also varies due to the presence of chlorinetogether with a predetermined quantity of a second radiation componentresulting from said irradiation which varies in intensity inversely withthe variations of said first radiation component due to the presence ofchlorine, said detecting means further comprising an output means forproviding a common output signal which varies as a function of thecombination of said first and second radiation components, and meansoperatively coupled to the output of said detecting means for utilizingsaid signal which varies as a function of the combination of said firstand second radiation components detected thereby and which variesprimarily as a function of said given characteristic to the substantialexclusion of the influence of chlorine which may also be present.

25. Apparatus as defined in claim 24 wherein said means for selectivelydetecting the combination of a first radiation component together with apredetermined quantity of a second radiation component comprises meansfor selectively detecting a first radiation component consistingessentially of neutron-induced gamma radiation together with apredetermined quantity of a second radiation component consistingessentially of thermal neutron radiation.

26. Apparatus as defined in claim 24 wherein said given characteristicis the hydrogen content of the irradiated formations.

27. Apparatus as defined in claim 25 wherein said given characteristicis the hydrogen content of the irradiated formations.

28. Apparatus as defined in claim 25 wherein said detecting meansincludes a scintillation type gamma radiation detector having associatedtherewith a pulse amplitude discriminator suitable to exclude gammaradiation below a predetermined energy level and which further includesmeans for rendering said radiation detector sensitive to thermal neutronradiation and wherein means are provided for adjusting the bias of saiddiscriminator.

29. Apparatus as defined in claim 24 wherein said means for selectivelydetecting the combination of a first radiation component together with apredetermined quantity of a second radiation component comprises meansfor selectively detecting a first radiation component which varies inintensity as a function of the hydrogen content of the irradiatedformations and which also varies due to the presence of chlorinetogether with a predetermined quantity of a second radiation componentresulting from said irradiation which varies in intensity inversely withthe variations of said first radiation component due to the presence ofchlorine and wherein said second radiation component also varies as afunction of the hydrogen content of the irradiated formations andwherein said variations in the intensity of said second radiation due tohydrogen are inverse to said variations in the intensity of said secondradiation due to the effect of chlorine.

References Cited by the Examiner UNITED STATES PATENTS Re. 24,38310/1957 McKay 2507l.5 2,933,609 4/1960 Norelius 250-83.6 3,090,8675/1963 Swanson et al. 25083.6

RALPH G. NILSON, Primary Examiner.

ARCHIE R. BORCHELT, Examiner.

7. IN APPARATUS FOR CONDUCTING A NEUTRON GAMMA RAY WELL LOG OF A BOREHOLE COMPRISING A LOGGING INSTRUMENT ADAPTED TO BE PASSED THROUGH A BOREHOLE AND COMPRISING A SOURCE OF NEUTRONS AND A DETECTOR OF GAMMA RAYSWHICH MAY RESULT FROM IRRADIATION OF AN EARTH FORMATION BY NEUTRONS FROMTHE SOURCE, THE IMPROVEMENT COMPRISING A SCINTILLATION TYPE GAMMA RAYDETECTOR HAVING A PREDETERMINED QUANTITY OF NEUTRON ABSORBING MATERIALPOSITIONED IN THE VICINITY OF THE DETECTOR AND CHARACTERIZED BY THEEMISSION OF RADIATION TO WHICH THE DETECTOR IS SENSITIVE, SAID DETECTORBEING SUBSTANTIALLY UNSHIELDED WITH RESPECT TO SAID RADIATION EMITTEDFROM SAID NEUTRON ABSORBING MATERIAL SUCH THAT SAID DETECTOR IS EXPOSEDTO AT LEAST A MAJOR PORTION OF SAID RADIATION EMITTED FROM SAID NEUTRONABSORBING MATERIAL AND TRAVELLING TOWARD SAID DETECTOR, THE QUANTITY OFSAID ABSORBING MATERIAL BEING CHOSEN IN RELATION TO THE INTENSITY ANDSPACING FROM SAID SOURCE SUCH THAT RADIATION WILL BE EMITTED THEREFROMIN RESPONSE TO INTERACTION WITH NEUTRONS HAVING AN INTENSITY WHICHSUBSTANTIALLY NEUTRALIZES THE EFFECT OF CHLORINE ON THE NEUTRON GAMMARAY LOG, SAID SCINTILLATION TYPE DETECTOR HAVING A PULSE AMPLITUDEDISCRIMINATOR ASSOCIATED THEREWITH WHICH IS BIASED TO EXCLUDE AT LEASTTHOSE LOWER ENERGY GAMMA RAYS WHICH MAY BE SCATTERED TO THE DETECTORFROM THE SOURCE.